1. Field of the Invention
The present invention relates to an aluminum-extruded multi-cavity flat tube having excellent brazing characteristics for use in an automotive heat exchanger (xe2x80x9caluminumxe2x80x9d herein used includes aluminum alloys). More specifically, the present invention relates to an aluminum-extruded multi-cavity flat tube used for an evaporator, condenser, and the like in an automotive air conditioner and a method for manufacturing the same.
2. Description of the Background Art and the Problems
Aluminum-made automotive heat exchanger components such as evaporators and condensers for use in automotive air conditioners are manufactured generally from an aluminum extruded multi-cavity flat tube and a fin that is fabricated into a corrugated shape with a brazing sheet comprising an aluminum core material clad with an Alxe2x80x94Si based brazing metal by assembling them into a prescribed shape and joining them by brazing.
In the above application, to destroy and remove aluminum oxides present on the surface of the aluminum-extruded multi-cavity flat tube as well as on the fin and to perform solid brazing, it is necessary to spray-coat the brazing surfaces with a flux suspended either in water or an alcohol after assembling the components into a desired configuration, evaporate the solvent, and thereafter perform the brazing procedure.
However, because of the complex structure of an aluminum-made automotive heat exchanger, it is often difficult to provide the surface of the flat tube or the fin with a uniform coating of the flux suspension, resulting in problems in which solid soldering is hindered at the positions where the flux coating is insufficient or, where the coating is excessive, the flux tends to fall off within a soldering furnace to contaminate or corrode the furnace and, at the same time, spoil the appearance of the processed products.
In recent years, to dispense with the flux coating procedure immediately before the brazing step, a method of coating flux beforehand on the surface of the material to be brazed, a method of coating with a mixed composition of a flux and a brazing metal, or brazing compositions for such uses have been proposed. (See Japanese Patent Application Laid-Open No. 35870/1991, Japanese Patent Application Laid-Open No. 285681/1994, Japanese Patent Publication of the Translation of International Patent Application No. 504485/1994, Japanese Patent No. 2681380, Japanese Patent No. 2681389, and others.)
These proposed methods have made it possible to apply a complete coating of flux on the surface of the material to be brazed, even with a complex design of the heat exchanger, dispense with the need to use costly brazing sheets in the case where a mixed composition of a flux and a brazing metal is applied and, at the same time, eliminate the problem of wear on the mold caused by the Alxe2x80x94Si based brazing metal during the fin molding step because an aluminum plate not clad with a brazing metal can be used.
While the coating step can be performed by a spray method, immersion method, roll-transfer printing, or the like, the spray method has problems such as an insufficient coating efficiency or clogging of the spray gun, whereas the immersion method has difficulty in performing a high-speed coating of a composition with a uniform formulation due to precipitation of the brazing metal powders or the flux powders. Accordingly, roll-transfer printing is the most efficient method suitable for mass manufacture in actual application.
However, among the organic resins to be evaporated during the heating step for the brazing, when a brazing composition comprising a resin binder such as an acrylic resin that consists of hydrocarbons of the ethylene series is applied with roll-transfer printing, as described in Japanese Patent Application Laid-Open No. 35870/1991, Japanese Patent No. 2681380, and Japanese Patent No. 2681389, it tends to create many locations where a brazing joint is not completely provided. Furthermore, when a flux composition comprising a surfactant for improving the wetability of the composition on an aluminum material (see Japanese Patent Application Laid-Open No. 285681/1994) is applied by roll-transfer printing, the surfactant acts to hinder the adhesion of the flux composition to cause the impaired brazing characteristics.
A flux composition using an alcohol such as isopropanol as an application medium (see Japanese Patent Publication of the Translation of International Patent Application No. 504485/1994) tends to cause precipitation of the brazing metal powders or the flux powders in the alcohol solution, leading to inferior performance in transfer printing and sometimes causing insufficient adhesion. Therefore, such a composition leaves a problem of inability to attain a solid brazing joint.
The present inventors have conducted extensive experiments and examinations for roll-transfer printing a flux composition or mixed compositions of flux and brazing metals on aluminum materials with a view to solving the above problems. As a result, the inventors have found that transfer printing performance and adhesion of a brazing composition or a flux composition to aluminum materials are significantly influenced by the type of synthetic resins in the composition, properties of organic solvents, and weight ratios of flux or brazing metals to synthetic resins in the compositions.
Based on the above findings, to achieve a continuous, speedy, and efficient coating application of a brazing flux composition and a mixed composition comprising flux and brazing metals on the surface of an aluminum extruded multi-cavity flat tube used in aluminum automotive heat exchanger components, such as evaporators and condensers, using a roll-transfer printing technique, the present invention has been achieved as a result of repeated experiments and examinations of a diverse nature conducted on synthetic resins in the composition, organic solvents, weight ratios of flux or brazing metals to synthetic resins in the composition, and their combinations.
Accordingly, an object of the present invention is to provide an aluminum extruded multi-cavity flat tube for use in automotive heat exchangers, such a tube being prepared with compositions exhibiting a satisfactory roll-transfer performance and adhesion and therefore exhibiting excellent brazing characteristics. Furthermore, another object of the present invention is to provide a method for manufacturing an aluminum extruded multi-cavity flat tube for use in automotive heat exchangers and having the above-described characteristics.
In accordance with the present invention, the aluminum extruded multi-cavity flat tube for use in automotive heat exchangers which achieves the above objects is characterized by a first feature whereby at least one of the flat surfaces of the tube is coated with a flux composition comprising a brazing flux and a synthetic resin based, as its main constituent, on a methacrylate homopolymer or a methacrylate copolymer. The aluminum extruded multi-cavity flat tube is further characterized by a second feature whereby the surface of the tube is coated with a brazing composition comprising a brazing flux, brazing metals, and a synthetic resin based, as its main constituent, on a methacrylate homopolymer or a methacrylate copolymer. The aluminum extruded multi-cavity flat tube is further characterized by a third feature whereby the weight ratio of the brazing flux to the synthetic resin in the flux composition, and the weight ratio of the total weight of brazing flux and brazing metals to the synthetic resin in the brazing composition are in the range from 9:1 to 7:3.
In accordance with the present invention, the method for manufacturing an aluminum extruded multi-cavity flat tube for use in automotive heat exchangers is characterized as follows:
1. At least one of the flat surfaces of an aluminum extruded multi-cavity flat tube is coated with a mixed flux composition comprising brazing flux powders added to an organic solvent in which a synthetic resin based, as its main constituent, on a methacrylate homopolymer or a methacrylate copolymer is dissolved using a roll-transfer printing technique, and subsequently heated or dried to evaporate the organic solvent in the mixed flux composition.
2. At least one of the flat surfaces of an aluminum extruded multi-cavity flat tube is coated with a mixed brazing composition comprising brazing flux powders and brazing metal powders added to an organic solvent in which a synthetic resin based, as its main constituent, on a methacrylate homopolymer or a methacrylate copolymer, is dissolved using a roll-transfer printing technique, and subsequently heated or dried to evaporate the organic solvent in the mixed brazing composition.
3. In the methods of 1 or 2 above, the organic solvent used is such that the atomic ratio of carbon to oxygen in the molecular structure of said organic solvent is a value between 2 and 3.
4. In the methods of 1 or 3 above, the viscosity of the mixed flux composition is between 100 and 10,000 mPa-s.
5. In any one of the methods of 1, 3 and 4 above, the weight ratio of the flux powders to the synthetic resin in the mixed flux composition is in the range from 9:1 to 7:3.
6. In the methods of 2 or 3 above, the viscosity of the mixed brazing composition is between 100 and 10,000 mPa-s.
7. In any one of the methods of 2, 3 and 6 above, the weight ratio of the total of the flux powders and the brazing metal powders to the synthetic resin in the mixed flux composition is in the range from 9:1 to 7:3.